Mmmmaybe they're childhood friends whose fathers met at work then! And he never realized she was a girl until much later but by that point, he's just trying to deny reality. Because he's an idiot.

AND/OR if you're okay with it, maybe we can also say one dad was working on the case of the other dad's murder? Then that dad dies (maybe because of the same murderer/criminal organization idk) and it's all one hot mess of blood and murder and gritty anime Batman-esque angst.